Do not show absurdedly high speeds on first update

We initialise both byte amounts to 0 and onCompleted can run earlier than
the first update of the model.


(cherry picked from commit 30b4a718)

applet/contents/ui/ConnectionItem.qml

@@ -216,8 +216,8 @@ PlasmaExtras.ExpandableListItem {
             prevTxBytes = TxBytes
         }
         onTriggered: {
-            rxBytes = (RxBytes - prevRxBytes) * 1000 / interval
-            txBytes = (TxBytes - prevTxBytes) * 1000 / interval
+            rxBytes = prevRxBytes == 0 ? 0 : (RxBytes - prevRxBytes) * 1000 / interval
+            txBytes = prevTxBytes == 0 ? 0 : (TxBytes - prevTxBytes) * 1000 / interval
             prevRxBytes = RxBytes
             prevTxBytes = TxBytes
         }